Mehul-

There are some remarkably accurate factory slugs loads out there which are designed specifically to run in smooth bore guns.

I'd take a look at the ammo end of the proposition first and see how well you can shoot your smooth bore gun with off the shelf ammo first.
That and I'd start with the cheaper Winchester etc stuff before I spent the bigger bucks on Brenneke slugs.
You may be in for a pleasant suprise.

My Mossberg 500 pump gun puts everything in a group smaller than my open hand at fifty yards. That's with the cheap Winchester two and a half inch rifled slugs. I never even tried the more expensive stuff. That's dead deer accurate within woods/iron sight range and better than good enough for me.

--Tinker